1. Field of the Invention
This invention relates to a frequency controlled phase locked oscillator.
2. Description of the Prior Art
The phase locked oscillator (PLO) disclosed in this application, has been developed in particular to meet the requirements of a magnetic disk file in which the position of data transducers are controlled during "read" and "write" operations by a closed loop servo system which derives servo position information from servo tracks previously recorded on an available disk surface in the file.
A function of the PLO is to supply clock pulses of an appropriate frequency to the "write" channel of the disk file to control "write" operations. Synchronization of the PLO is achieved by controlling its frequency so as to be in phase with servo clock pulses derived from the servo tracks. Since the clock pulse frequency supplied to the write channel is considerably greater than the servo clock frequency, the actual synchronizing process is conducted by using a signal which is a subharmonic of the actual PLO signal. The clock pulses supplied by the PLO to the "write" channel are required to be of a stable frequency, and ideally there should be no short term variations in frequency due to slight misalignment of the servo clock pulses on the disk. Essentially this requirement means that the PLO should have a low bandwidth only responding to the long term variations produced by speed changes of the disk, for example.